RLS causes a wide range of uncomfortable leg sensations. They tend to occur most often when the legs are at rest during the day or in the evening. The sensations are almost always accompanied by an irresistible need to move the legs. Moving the legs can bring temporary relief.

Periodic limb movement disorder (PLMD) causes people to kick and jerk their arms and legs throughout the night. In PLMD, leg and arm muscles may involuntarily contract hundreds of times a night. You may not be aware of it, but a bed partner probably will be.

PLMD limb movements may last only a few minutes or may continue for hours, with intervals of sound sleep in between. They usually cluster in the first half of the night and occur mainly during non-REM (non-dreaming) sleep.

People with PLMD generally awaken for a few seconds at a time during limb movements, often without realizing it. Then they fall back into the lighter stages of sleep. Unless a bed partner complains, people with PLMD are often oblivious to their movements. They may wake up baffled at why they feel exhausted despite getting what they thought was a full night's rest.

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Most people with RLS also have PLMD — but the reverse is not true. And the two disorders have several key distinctions. For example, RLS occurs mainly while awake but at rest; PLMD occurs during sleep. RLS involves voluntary movements performed to ease discomfort. PLMD involves involuntary movements that the person may not even be aware of. (I've put a more detailed chart explaining the differences between RLS and PLMD on my website, AskDoctorK.com.)

Many people with RLS have iron deficiency. Taking iron supplements treats not only the iron deficiency, but also improves the symptoms of RLS. You should not take iron supplements unless blood tests have proved that you have iron deficiency. That's because some people have a genetic condition that leads to iron overload if they take iron supplements they don't need.

Also, if you have RLS and are found to have iron deficiency, your doctor should diagnose why you have it. The most common cause is menstrual bleeding. But sometimes a person can be very slowly losing blood (and the iron in the blood) without knowing it. For example, it could be from cancer in the stomach or colon that is not yet causing any symptoms.

There's some evidence that exercise can ease both RLS and PLMD. Walking or other moderate exercise such as biking or swimming are good choices. Some people find that cold showers help; others prefer heat.

Drugs that ease the tremors of Parkinson's disease also reduce the number of leg movements. They can improve quality of life if you have RLS or PLMD.